In conventional manner, a polyphase alternator comprises a rectifier bridge constituted by a set of negative end diodes and by a set of positive end diodes. The positive end diodes are mounted on a positive dissipator, while the negative end diodes are mounted directly in the back plate, which acts both as a connection to ground and as a dissipator.
There are two main types of mounting for such diodes: press-fit and soldering. When negative end diodes are to be mounted in a metal back plate, e.g. made of aluminum alloy, however, soldering is out of the question. Given the heat capacity of the plate, such mounting would require an unreasonable amount of heat to be applied. The diodes must therefore necessarily be press-fit diodes, and that is disadvantageous, in particular because of the need to use a press, or because diode mounting then requires two different techniques to be used: soldering for positive diodes and pressing for negative diodes.
One solution to this problem consists in providing a negative dissipator onto which the negative end diodes are soldered, with the dissipator being fitted to the back plate.
That solution is also disadvantageous since it requires a special design of back plate plus an additional part, i.e. the fitted-on negative dissipator, and it increases the axial size of the alternator.
It results from the above discussion that depending on the technique used for mounting negative diodes, it is necessary to resort to two back plates of different types, which is also disadvantageous, in particular for standardizing manufacture.